provide description on dead letter topics

Author: hernandezc1

broker/setup_broker/lvk/README.md

@@ -1,6 +1,6 @@
 # Connect Pitt-Google to the LIGO/Virgo/KAGRA (LVK) Gravitational Wave Alert Stream
 
-April 2024 - Author: Christopher Hernandez
+Updated January 2025 - Author: Christopher Hernández
 
 - [Overview](#overview)
 - [Setup](#setup)
@@ -37,8 +37,10 @@ You may want to
 or
 [set up a GCP project from scratch](https://pitt-broker.readthedocs.io/en/latest/broker/run-a-broker-instance/initial-setup.html#setup-local-environment).
 
-[Create secrets](https://cloud.google.com/secret-manager/docs/creating-and-accessing-secrets#create) for your client ID
-and client secret
+
+### Use GCP's Secret Manager
+[Secret Manager](https://cloud.google.com/secret-manager/docs/overview) is a service that allows users to manage and store sensitive data. Use the following code snippet to [create secrets](https://cloud.google.com/secret-manager/docs/creating-and-accessing-secrets#create) for your client ID
+and client secret. This information will be used to deploy a broker instance with your client credentials.
 
 ```bash
 # define parameters
@@ -50,7 +52,6 @@ client_id="${survey}-${PROJECT_ID}-client-id"
 client_secret="${survey}-${PROJECT_ID}-client-secret"
 
 # create secret(s)
-
 gcloud secrets create "${client_id}" \
     --replication-policy="automatic"
 gcloud secrets create "${client_secret}" \
@@ -84,6 +85,10 @@ gcloud secrets add-iam-policy-binding "${client_id}" --member="serviceAccount:${
 gcloud secrets add-iam-policy-binding "${client_secret}" --member="serviceAccount:${user}" --role="${roleid}"
 ```
 
+### BigQuery subscriptions
+Our broker uses [BigQuery subscriptions](https://cloud.google.com/pubsub/docs/bigquery) to write alert data directly to a BigQuery table.
+Messages receieved by BigQuery subscriptions that fail to write data to BigQuery are negatively acknowledged and re-sent. If the messages fail enough times (default number of attempts = 5), then the message is subsequently moved to a [dead letter topic](https://cloud.google.com/pubsub/docs/handling-failures#dead_letter_topic). A subscription to this dead letter topic is automatically made, allowing the user to identify which messages failed to write data to BigQuery and why the write operation failed.
+
 ## Deploy broker instance
 
 Clone the repo and cd into the directory:
@@ -110,6 +115,8 @@ complete, the VM will shut down automatically. You can check the status of the V
 [Google Cloud Console](https://console.cloud.google.com/compute).
 This entire process should take less than 10 minutes.
 
+
+
 ## Start the Consumer VM to ingest the LVK alert stream
 
 ```bash